How do you implement a subscription using PubSub in NestJS GraphQL?

Q: How do you implement a subscription using PubSub in NestJS GraphQL?

// 1. Register PubSub as a provider export const PUB_SUB = 'PUB_SUB'; @Module({ providers: [ { provide: PUB_SUB, useValue: new PubSub() }, PostsResolver, ], }) export class PostsModule {} // 2. Resolver const POST_CREATED_EVENT = 'postCreated'; @Resolver(() => Post) export class PostsResolver { constructor( private postsService: PostsService, @Inject(PUB_SUB) private pubSub: PubSub, ) {} // Mutation publishes the event after the write @Mutation(() => Post) async createPost(@Args('input') input: CreatePostInput): Promise { const post = await this.postsService.create(input); await this.pubSub.publish(POST_CREATED_EVENT, { postCreated: post }); return post; } // Subscription subscribes to the event stream @Subscription(() => Post) postCreated() { return this.pubSub.asyncIterator(POST_CREATED_EVENT); } } Register PubSub with an injection token — inject it into resolvers via @Inject(PUB_SUB). The object key in publish() must match the subscription field name: { postCreated: post }. asyncIterator() argument must match the event name used in publish(). In-memory PubSub works only in single-process deployments — use Redis PubSub for multiple instances. Publish after the database write completes — never publish on a failed or rolled-back write.

Difficulty: 5/10

GraphQL Subscriptions, NestJS PubSub, Resolver design

Register PubSub as a provider using an injection token. In the mutation, publish the event after the write. In the subscription resolver method, return pubSub.asyncIterator() with the event name. The @Subscription() return type arrow function and the event payload key must match the asyncIterator event name.

PubSub subscription with publish and subscribe

PubSub subscription rules:

1
Register PubSub with an injection token — inject it into resolvers via @Inject(PUB_SUB).
2
The object key in publish() must match the subscription field name: { postCreated: post }.

How do you implement a subscription using PubSub in NestJS GraphQL?

Difficulty: 5/10

GraphQL Subscriptions, NestJS PubSub, Resolver design

Register PubSub as a provider using an injection token. In the mutation, publish the event after the write. In the subscription resolver method, return pubSub.asyncIterator() with the event name. The @Subscription() return type arrow function and the event payload key must match the asyncIterator event name.

PubSub subscription with publish and subscribe

PubSub subscription rules:

1
Register PubSub with an injection token — inject it into resolvers via @Inject(PUB_SUB).
2
The object key in publish() must match the subscription field name: { postCreated: post }.

How do you implement a subscription using PubSub in NestJS GraphQL?

How do you implement a subscription using PubSub in NestJS GraphQL?

Scenario Questions

Follow-up Questions